The present invention relates generally to microelectronic devices and fabrication methods therefor, and, more particularly, to light-emitting devices and fabrication methods therefor.
Light-emitting diodes (LEDs) are widely used in consumer and commercial applications. As is well known to those skilled in the art, a light-emitting diode generally includes a diode region on a microelectronic substrate. The microelectronic substrate may comprise, for example, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide, and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new display applications, and may place LEDs in a position to compete with well entrenched incandescent and fluorescent lamps.
Referring now to FIG. 1, a conventional GaN-based LED 100 comprises a SiC substrate 105 that has first and second opposing surfaces 110a and 110b, respectively, and is at least partially transparent to optical radiation. A diode region, comprising an n-type layer 115, an active region 120, and a p-type layer 125 is disposed on the second surface 110b and is configured to emit optical radiation into the SiC substrate 105 upon application of a voltage across the diode region, for example across ohmic contacts 130 and 135.
The diode region including the n-type layer 115, the active region 120, and/or the p-type layer 125 may comprise gallium nitride-based semiconductor layers, including alloys thereof, such as indium gallium nitride and/or aluminum indium gallium nitride. The fabrication of gallium nitride on silicon carbide is known to those skilled in the art, and is described, for example, in U.S. Pat. No. 6,177,688, the disclosure of which is hereby incorporated herein by reference. It will also be understood that a buffer layer or layers comprising aluminum nitride, for example, may be provided between the n-type gallium nitride layer 115 and the silicon carbide substrate 105, as described in U.S. Pat. Nos. 5,393,993, 5,523,589, 6,177,688, and application Ser. No. 09/154,363 entitled Vertical Geometry InGaN Light Emitting Diode, the disclosures of which are hereby incorporated herein by reference.
The active region 120 may comprise a single layer of n-type, p-type, or intrinsic gallium nitride-based materials, another homostructure, a single heterostructure, a double heterostructure, and/or a quantum well structure, all of which are well known to those skilled in the art. Moreover, the active region 120 may comprise a light-emitting layer bounded by one or more cladding layers. The n-type gallium nitride layer 115 may comprise silicon-doped gallium nitride, while the p-type gallium nitride layer 125 may comprise magnesium-doped gallium nitride. In addition, the active region 120 may include at least one indium gallium nitride quantum well.
In some LEDs, the ohmic contact 135 for the p-type gallium nitride layer 125 comprises platinum, nickel and/or titanium/gold. In other LEDs, a reflective ohmic contact comprising, for example, aluminum and/or silver, may be used. The ohmic contact 130 to the n-type gallium nitride layer 115 may comprise aluminum and/or titanium. Other suitable materials that form ohmic contacts to p-type gallium nitride and n-type gallium nitride may be used for ohmic contacts 135 and 130, respectively. Examples of ohmic contacts to n-type gallium nitride layers and p-type gallium nitride layers are described, for example, in U.S. Pat. No. 5,767,581, the disclosure of which is hereby incorporated herein by reference.
Unfortunately, the majority of light that is generated inside of an LED device typically never escapes the device because of various optical losses, such as total internal reflection (TIR). Referring now to FIG. 2, when light travels from one medium to another, it may be refracted such that the angle of refraction is governed by Snell's law as follows: n1 sin θ1=n2 sin θ2, where n1 is the index of refraction for medium 1 and n2 is the index of refraction for medium 2. The light that escapes, however, has an angular dependence that is less than the “critical angle,” which is defined as follows θ1critical=sin−1(n2/n1). Light that is incident at an angle greater than the critical angle does not pass through to medium 2, but is instead reflected back into medium 1. This reflection is commonly called total internal reflection. Thus, mediums having significantly different indices of refraction may result in a relatively small critical angle for light transmitted through the two mediums and may result in significant optical loss due to total internal reflection.